1. Field of the Invention
The present invention relates to a lithographic projection apparatus.
2. Description of the Related Art
As used herein the term xe2x80x9cpatterning devicexe2x80x9d should be broadly interpreted as referring to a device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The use of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a pattering device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an array is that addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described above, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A still further example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning devices as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (IC). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clens.xe2x80x9d However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory procedures may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In a lithographic apparatus, it is common to purge the beam path and the vicinity of the mask and substrate to provide a controlled atmosphere, the aim being to prevent contamination of the mask and substrate and scattering of the projection beam by dust particles and also to provide a constant and consistent atmosphere for interferometric measuring devices which measure the position of the substrate and mask tables. Where the projection beam is of a short wavelength, for example 157 nm or 126 nm, having a relatively high absorption in air, the gas used for the purge may be nitrogen of high purity. Suitable compositions of such a purge gas are disclosed in EP-1 172 698-A, which document is hereby incorporated by reference. Apparatus using nitrogen to purge the beam path and mask or substrate stages are particularly sensitive to leakage of air into the controlled atmosphere since this will result in a change in refractive index, disturbing the interferometric sensors.
In some arrangement using gases to control contamination, nested compartments may be provided with the innermost compartment containing the component to be protected at a higher pressure, for example as shown in WO 99/57607. This ensures that any leakage is away from the component to be protected. Conversely, in vacuum systems, it is known to have a series of compartments with the inner compartment at lower pressures, see for example EP-1 052 551-A2 and EP-0 532 968-A1. This reduces the pressure differentiates across, and hence forces on, vulnerable parts of the vacuum compartment, e.g. windows.
To reduce the consumption of purge gas, which is expensive due to its purity, in particular in scanners, it has been proposed to provide compartments around the mask and/or substrate tables which move with the respective table as the table scans. This avoids the need to purge a larger compartment encompassing the entire range of movement of the table. Such moveable purge compartments are disclosed in EP-1 098 225-A, which document is hereby incorporated by reference. To prevent contamination by leaks of external air into the purge compartment, the purge compartment is maintained at slight over-pressure, e.g. of a few Pa, relative to the remainder of the apparatus.
However, in a scanner, the mask and substrate tables are moved at high velocities and with high accelerations. During both acceleration and constant velocity movement, the pressure within a moving purge chamber is not uniform or constant. In particular, at the leading edge of the chamber significant pressure drops can occur leading to this part of the purge compartment being at a lower pressure than the rest of the apparatus so that inward leaks are possible. Conversely, the pressure at the trailing edge can rise substantially leading to increased leakage of purge gas into the vicinity of the beam paths of the interferometric measuring devices disturbing the reflective index there.
It is an object of the present invention to provide a moveable purge compartment in which inward and/or outward leakage is avoided or reduced even when the purge compartment is undergoing large accelerations or moving at high velocity.
This and other objects are achieved according to the invention in a lithographic projection apparatus including a radiation system for supplying a projection beam of radiation; a support structure for supporting patterning device, the patterning device serving to pattern the projection beam according to a desired pattern; a substrate table for holding a substrate; a projection system for projecting the patterned beam onto a target portion of the substrate; and a first compartment at least partially enclosing and moving with a moveable component of said apparatus; and a second compartment at least partially enclosing and moving with said first compartment and a purge gas system for supplying purge gas to said first compartment and removing purge gas from said second compartment such that said first compartment is, in use, at an average pressure P1, said second compartment is, in use, at an average P2, whereby the pressures P1, P2 satisfy the inequalities: P1 greater than P0 and P2 less than P0, where P0 is the pressure of the environment exterior to the second compartment.
Even when acceleration of the moveable object and compartments cause local pressure variations, the first or inner compartment is at a higher pressure than the second or outer compartment so that any gas flow is outward and contamination is prevented from reaching the inner compartment. Also, the second compartment can be maintained at a lower pressure than the remainder of the apparatus, particularly the beam paths of any interferometric displacement measuring devices, so that leakage of purge gas to disturb these devices is minimized.
Preferably, the depth of the second compartment is substantially equal to that of the first compartment so that pressure variations caused by movements are the same in the two compartments. Isolation of the inner compartment can be further improved by providing a plurality of second compartments nested about the first compartment. In such an arrangement, the pressure of the outermost second compartment is preferably lowest.
According to a further aspect of the invention there is provided a device manufacturing method using a lithographic apparatus, the method including providing a substrate that is at least partially covered by a layer of radiation sensitive material; providing a projection beam of radiation using a radiation system; using patterning device to endow the projection beam with a pattern in its cross-section; projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material, providing a first compartment at least partially enclosing and moving with a moveable component of said apparatus; providing a second compartment at least partially enclosing and moving with said first compartment; and supplying purge gas to said first compartment and removing purge gas from said second compartment such that said first compartment is at an average pressure P1 and said second compartment is at an average pressure P2; whereby the pressures P1, P2 satisfy the inequalities P1 greater than P0 and P2 less than P0, where P0 is the average pressure of the environment exterior to said second compartment.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. One of ordinary skill in the art will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms radiation and beam are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.